High-frequency heating applicators



Feb. 26, 1957, H. R. WARREN HIGH-FREQUENCY HEATING APPLICATORS 3 SheetS-Sheet 1 Filed March 26. 1954 ,w a a Conveyor Travel Feb. 26, 1957 H.R. WARREN HIGH-FREQUENCY HEATING APPLICATORS 3 Sheets-Sheet 2 FiledMarch 26, 1954 Feb. 26, 1957 H. R. WARREN HIGH-FREQUENCY HEATINGAPPLICATORS 3 Shets-Sheet 3 Filed March 26. 1954 Fly. 7

U. I I I I I I I I IH H I I I I F I m w m E I! u m E I. I E E E I 5 m MII I I I IHHHHI I m w% H D II m A W D D I D D I m 2 a m I F/ga UnitedStates Patent HIGH-F REQUEN CY HEATING APPLICATORS Henry R. Warren,Columbus, Ind., assignor to National Cylinder Gas Company, Chicago,ill., a corporation of Delaware Application March 26, 1954, Serial No.419,072 16 Claims. (Cl. 219-1055) This invention relates tohigh-frequency heating and particularly to resonant applicatorsespecially suited for rapid dielectric heating of large area loads, suchas foamrubber mattresses, wallboard panels, groups of said cores orplastic preforms, and the like. i

This aplication, a continuation-in-part of my application Serial No.138,628, filed January 14, 1950, and now abandoned in favor of mycontinuation-in-part application Serial No. 419,633, filed March 26,1954, has claims directed to subject matter divided from my parentapplication Serial No. 138,628. My aforesaid applications contain a moredetailed discussion, included herein by this reference, of thestructural and operating characteristics of resonant applicators of thekind to which the present invention is particularly directed.

In general, such applicators comprise relatively large electrodestructures electrically interconnected through conductive structurewhich at least in part has substantial inductance cooperative withcapacity-means including the capacitance between said electrodestructures to form a resonant circuit device, and a power transfercoupling loop disposed in position to be traversed by a high-frequencymagnetic field encircling a part of said interconnecting conductivestructure. More particularly, in the preferred form of such applicators,the interconnecting structure includes low-resistance conductive wallsof a shielding enclosure which completes the resonant circuit of saiddevice and serves to confine said magnetic field and also the electricfield produced between said electrode structures, at least one of theelectrode structures being electrically interconnected with wallstructure of the enclosure through a leg or fin element whichreentrantly projects into the enclosure and in which a substantial partof the inductance of the resonant applicator is concentrated, and thesaid coupling loop being arranged to be traversed by the magnetic fieldencircling the inwardly projecting leg or fin element. Such loop mayserve to provide for excitation by an oscillator having the loop in itsanode circuit.

In accordance with the present invention, the mutual inductance of theapplicator and coupling loop is determined by conductive structure whichcontrols the magnetic flux distribution or density in the vicinity ofthe coupling loop. Such conductive structure may, in whole or in part,be stationary to serve as a flux guide or it may, in whole or in part,be movable to serve as a flux gate.

In most cases, particularly when the applicator is thefrequency-determining element of an oscillator system, it is preferablethat the flux-controlling structure be constructed and arranged so as toobtain a flux distribution in the vicinity of the loop such that themutual inductance will be in the supraoptimum range as taught andbroadly claimed in my aforesaid parent applications and as hereinafterdiscussed in detail. More particularly, when in such cases the variationin mutual inductance is eifected by movement of flux gate structure, orby variation of the effective area of the loop, or by both, suchvariation 2,783,346 Patented Feb. 26, 1957 should be entirely within thesupraoptimum range for stabilization of the heating electrode voltage.On the other hand, when only flux guide structure is provided, theeffective loop area may be variable to adjust the electrode voltage, astaught in my aforesaid parent applications, certain features of whichteachings are more particularly set forth and claimed in my applicationSerial No. 419,074, filed March 26, 1954, as a divisionalcontinuation-in-part of said application Serial No. 138,628. The fiuxguide structure is preferably used to define a limit of variation ofmutual inductance which is entirely within the supraoptirnum range.

The invention further resides in features of construction, combinationand arrangement hereinafter described and claimed.

For a more detailed understanding of the invention and for illustrationof various embodiments thereof, reference is made to the accompanyingdrawings, in which:

Fig. 1 is a sectional view of a resonant applicator and associatedoscillator system;

Fig. 2 is a top plan view, in part broken away, of the applicator ofFig. 1;

Fig. 2A is a perspective schematic view of a flux-gate arrangement andoperating mechanism therefor;

Figs. 3 and 4 are top plan and elevational views, in part broken awayand partly in section, of a modification of the applicator of Figs. 1,2;

Figs. 5 and 6 are top plan and elevational views, in part broken awayand partly in section, of another modification; and

Figs. 7 to 10 are top plan views, in part broken away, of furthermodifications.

in the embodiment illustrated in Figs. 1 and 2, the reentrant resonantapplicator 10A comprises a housing 11A of sheet metal, or the like,forming a tunnel with conductive walls. The heating electrode 16A whichmay be of substantial length and width, is disposed within the tunnel inspaced relation to the wall structure thereof. This upper electrodehereinafter referred to as the hot electrode, is supported from andconnected to the upper wall of the housing 11A by a reentrantlyextending conductive fin structure 13A. The bottom wall of the tunnelmay serve as the cold electrode 15A. Alternatively, the lower electrodemay be an auxiliary conductive member, movable or stationary,conductively connected or otherwise coupled to the tunnel wallstructure. In such latter cases, the connection or coupling of the lowerelectrode to the wall structure may be eifected through a second finstructure, or equivalent, as in certain of the constructions disclosedin my aforesaid parent applications.

In the illustrated construction, all, or practically all, of theinductance of the resonant applicator is concentrated in the conductivefin 13A and all, or practically all, of the capacitance of theapplicator is that between the electrodes 15A, 16A, so that the resonantfrequency of the applicator is predominantly determined by theinductance of the fin and the capacitance between the electrodes. In theapplicator as thus far described, the wall structure of housing 11Aserves as a path of very low resistance and very low reactance for thecirculating current between the lower heating electrode 15A and theupper end of fin 13A.

Because of the low resistance of the fin, of the heating electrodes andof the wall structure, the unloaded Q of the applicator is very high.Because of their high-frequency, the circulating currents arepractically confined to the inner surfaces of the applicator housing ortunnel and consequently all external surfaces are at ground potentialand serve as a radio-frequency shield for the internal components whichare at high radio-frequency poparent applications.

tential. As a result, the radiation losses are low, which minimizesradio interference and further contributes to high Q of the tunnel somaking it uniquely suitable for dielectric load materials having verylow power-factor, such as foam rubber, extruded rubber hose or gaskets,

as well as for materials having high power factor, such and which may beclosed to minimize interference to sensitive radio-receiving equipment.Partial end closures, leaving an unobstructed path communicating withthe interelectrode space for insertion or removal of work from eitherend of the housing or for continuous flow of work, may be provided inmanner discussed in said When the tunnel has end walls, partial orcomplete, the edges or sides of the fin inductor must be spacedtherefrom to leave an unobstructed path about the fin inductor for thefin flux, or high-frequency magnetic field.

The resonant applicator 19A, like others hereinafter i described, isinductively coupled to a high-frequency exciting source by'apower-transfer loop 51A which is disposed within the tunnel and threadedby the high-frequency magnetic field or flux encircling the fin inductor13A. The loop 51A is preferably and usually a single turn loop. Thisloop is adjustable, as by control knob 66A having an insulated shaft65A, to vary its effective area normal to the flux lines, so to permitsmooth variation of the mutual inductance of the resonant applicator Yand the coupling loop. For reasons later herein more fully discussed,the range of adjustment of mutual inductance, however effected, ispreferably entirely in the supraoptimum range.

The fin structure 13A is offset laterally with respect to the applicatorhousing 11A to accommodate the shielding enclosure or compartment 64Awhich houses the oscillator tube 25A and associated circuit components.

The loop 51A extends from shielding compartment 64A couple the resonantapplicator to the anode circuit of the tube. a

The shielding compartment 64A also serves as a flux baifie or guidewhich produces a localized distortion of.

the magnetic field so as to increase the flux density in the vicinity ofthe coupling loop 51A and thereby increasing the mutual inductance for agiven size loop. In effect,

and as best shown in Fig. 2, the front and side walls 18A, 20A, 20A'ofcompartment 64A serve as an inwardly recessed or concave side wall ofhousing 11A which constricts the flux path between the fin 13A and thewall from which loop 51A extends, thereby producing a concentration ofthe flux in the vicinity of the loop. Thus the mutual inductance is muchgreater than if housing 64A were exterior of housing MA with loop 51Aextending into the correspondingly greater flux space.

- If for any reason, it is desired to provide flux path reeffective areaas presented to the flux lines. Alternatively, or in addition, forgreater range of electrodevoltage control, thefluX-guide afforded byfront wall 18A of compartment 64A may be provided with an gularlyadjustable metal plates 21A (Fig. 2) which serve 4 into the magneticflux path of the fin structure 13A to as flux gates and are operable tovary the ratio between the efiective area of the loop and thecross-sectional area of the flux path in the vicinity of the loop. Thatcrosssectional area is the area subtended by the loop 51A between saidinductance element 13A and the opposing wall structure 18A of thehousing10A. Increasing such ratio increases the mutual inductance between theloop and the applicator. The gates may be solid or perforate.

As gates 21A are swung from the position shown toward flux guide 18A,more and more of the available fin flux is directed through the area ofloop 51A. The mutual inductance is thus increased so to effect areduction in potential of electrode 16A-assuming the mutual inductanceis in the supraoptimum range. Conversely, as gates 21A are swung towardfin 13A, there results a localized distortion of the flux path such thatmore and more of the magnetic flux is diverted from the loop area withconsequent decrease in the mutual inductance coupling and increase ofthe heating electrode potential. The gates 21A are preferably ganged foradjustment in unison as by external mechanical coupling betweenactuating shafts extending through the top wall of housing 11A. Sucharrangement, schematically illustrated by mechanism 22A, Fig. 2A, mayalso be used in subsequently described modifications.

For reasons fully discussed in my aforesaid parent applications, it isof particular advantage in dielectric heating to maintain the mutualinductance in the supraoptimum range throughout its variation byadjustment of loop 51A and/ or the flux gates 21A. It sufiices here tostate that under such circumstance, the heating electrode voltage for agiven setting of loop 51A and of the flux gates 21A does not excessivelyvary with change in loaded of the applicator as would otherwise occur(in absence of supraoptimum coupling) upon change of the electricalcharacteristics of a dielectric load during its heating or, in aconveyor-fed applicator, of change of the number of load objects movingbetween the heating electrodes.

Generally speaking, to obtain high-electrode voltages suited for heatingof low power-factor loads, the resonant frequency of the anode-loopcircuit should be higher than and non-harmonically related to theresonant frequency of the applicator, and consequently the size of theanode loop must be restricted to meet this requirement. On the otherhand, to obtain supraoptimum coupling, the loop area must besufliciently large to be traversed by a substantial percentage of theflux encircling the fin inductance. By use of flux guide 18A, or itsequivalent, supraoptimum coupling may be obtained with a relativelysmall loop despite large resonator dimensions and other designconsiderations required for heating of large area loads. 1

The oscillator tube 25A is shown in Fig. 1 as included in a preferredoscillator system, more fully described and claimed in myv aforesaidparent applications, wherein the frequency of oscillations produced bythe oscillator system is predominantly determined by the resonantapplicator which forms an element of such system. The tube 25A and theassociated components comprise a high-frequency power supply meanshaving a powerdelivery circuit which includes the power-transfer loopdisposed within the unobstructed path of the magnetic field encirclingthe inductance element 13A for a transfer of such high-frequency powerto the power-receiving circuit comprising the resonant applicator 10A.Briefly, the grid-excitation voltage of oscillator tube 25A is derivedfrom the heating-electrode voltage and in a manner insuring that thepercentage relation between these voltages varies' automatically withthe heating load to stabilize the voltage of the heating electrode.Specifically, the grid of tube 25A may be connected to the heatingelectrode 16A by a capacitor 59A whose reactance;usual- 1y is highrelative to the reactance of the effective input wa es capacity of thetube exemplified by capacitor 62A. The cathode of tube A, so far as thegenerated oscillations are concerned, is grounded through theby-pass'condensers 61A.

The radio-frequency potential of the grid of tube 25A is thus always afraction of the radio-frequency potentialdifference between the heatingelectrodes and is inversely proportional to the ratio of the totalreactance of the series-connected capacitors 59A, 62A to the reactanceof the effective input capacity 62A of tube 25A.

With loop 51A and/or the flux gates 21A adjusted to provide the desiredradio-frequency voltage on heating electrode 16A, or equivalent, andwith capacitor 59A preadjusted or preset for proper grid-excitation, thecapacity 62A has an effective value which, as fully explained in myaforesaid parent applications, inherently varies with applicator load sothat the ratio of the reactances of the capacity voltage-divider 59A,62A varies automatically with load and in proper sense to stabilize thegrid-excitation.

With such preferred oscillator arrangement, including supraoptimumcoupling, there is essentially uniform heating of the load regardless ofvariations in the load density (i. e., number of size of objects beingheated), whereas otherwise the heating rate would vary greatly with loaddensity causing arcing and excessive grid-excitation to occur at lightloads. With the mutual inductance preadjusted to provide the desiredheating-electrode voltage, and when both the potential-divider 59A, 62Aand supraoptimum coupling are incorporated, the grid current remainspractically constant over a wide range of load-circuit power-factors, soeliminating need for constant supervision by an operator, or byauxiliary controls, to insure safe and efiicient operation of theoscillator tube as well as safe and uniform heating of the load.

It is to be understood that all of the foregoing is also applicable tothe modifications hereinafter described. For brevity, correspondingcomponents are identified by the same reference numbers but with lettersuffixes identifying the particular modification.

With the offset fin-electrode arrangement shown in Fig. 1, work objectsA19, 19A, of different heights may be heated without raising or loweringof the heating electrodes 16A. However, for most uses, it is desirable,as shown in Figs. 3 to 10, that provision be made for varying theelectrode spacing to accommodate work objects of diiferent height or tovary the spacing between the work and the hot electrode.

Referring to Figs. 3 and 4, the hot electrode 163 of resonant applicaton1013 may be raised and lowered by rods 173 which, as more fullydescribed and claimed in my copending application Serial No. 419,070,filed March 26, 1954, are disposed within the hollow fin inductance 13B.The rods 17B may be moved vertically by any suitable mechanism.

In this modification, the flux-guide 18B is an auxiliary conductivestructure or plate supported by insulators be tween the coupling loop513 and the adjacent side wall of the applicator housing 11B. The fiuxgates 21B are pivotally mounted on flux-guide 13B in the vicinity of thecoupling loop and on opposite sides thereof for variably distorting theflux path so as to effect adjustment of the mutual inductance. As theflux gates 21B are swung from the position shown (Fig. 3) toward the fin13B, more rot" the high-frequency magnetic flux of the fin is divertedfrom that region of the space between the fluxguide and the fin in whichthe coupling loop is disposed, so to decrease the mutual inductance.Conversely, as the flux gates 21?- are swung toward the supporting Wallof housing 113, more of the total fin flux is concentrated through theloop space between flux-guide 13B and fin 13B so as to increase themutual inductance.

In the modification shown in Figs. 5 and 6, the fluxguide 18C is anauxiliary conductive structure or plate supported by insulators betweenthe coupling loop 510 site sides of the and the fin structure 130. Theflux gates 21C are mounted dnl the flux-guideilfiC and are respectivelyon opposite sides of the coupling loop 51C. As the flux gates 21C areswung from the position shown (Fig. 5) toward the fin inductor 13C, moreof the high-frequency magnetic flux of the fin is directed into thespace between the fiux-guide 18C and the adjacent wall of housing 11C soto increase the mutual inductance of the applicator and the couplingloop. Conversely, as the flux gates are swung away from the fin 13C,more of the flux is diverted from the coupling loop space and into thespace between the flux-guide 18C and the fin 13C so to decrease themutual inductance.

In the modification shown in Fig. 7, the flux gates 21D are pivotallymounted on the fin structure 13D on oppocoupling loop 51D. As the fluxgates 211') are swung away from the position shown toward the tin 13D,more of the fin flux traverses the coupling loop and so increases themutual inductance. As the flux gates 2113 are swung away from theposition shown toward the side wall of applicator housing 11D, more ofthe fin flux is diverted from the coupling loop with consequent decreaseof the mutual inductance.

in the modification shown in Fig. 8, the flux gates 21E are pivotallymounted on a side wall of applicator housing 1135 in the vicinity ofcoupling loop 51E and on opposite sides thereof. As the flux gates 21Bare moved away from the position shown toward the fin inductor 13E, moreof the high-frequency magnetic flux of the fin is diverted from thecoupling loop so to decrease the mutual inductance. Conversely, as theflux gates 21B are swung toward the applicator Wall, more of the finflux traverses the coupling loop 51E so increasing the mutualinductance.

in the modification shown in Fi 9, the flux-guide 18F is supported byinsulators between the coupling loop 51F and fin inductor 13F and is ofsuch length that part of the total fin flux is always diverted into thespace between the flux-guide 18F and the fin 13F correspondingly toreduce the value of mutual inductance otherwise existing between theresonant applicator and the coupling loop. By selection of the length ofthe flux-guide and/or its spacing from the fin, the percentage of finflux diverted can be adjusted to obtain the desired mutual inductancewithout redesign or reconstruction of the loop, applicator housing andfin arran ement.

For variation of the mutual inductance, the effective loop area may bevaried by any suitable arrangement including those disclosed in myaforesaid application Serial No. 419,074, and in my said parentapplications or the cross-sectional area of the flux space between thefin structure and the opposite housing wall may be distorted or variedby flux gates mounted on the guide, generally as in Fig. 5, or upon thetunnel wall, generally as in Fig. 8. Either such expedient is effectiveto afford variation in the ratio between the effective area of the loopand the cross-sectional area of the fiuxspace subtended by the loop. Thelast-named flux space is the space between the tin structure and theopposing wall structure (whether that wall structure be the housing wall11D of Fig. 7 or whether it be the compartment wall 18A of Fig. l), withupper and lower limits corresponding with those of the loop.

In the modification shown in 10, the box fin inductor 13G is offset withrespect to the applicator housing 116 so that the flux density isgreater in that region of the flux space in which the loop is disposedthan elsewhere around the fin. Thus for given dimensions of the loop,fin inductor and applicator housing, the mutual inductance is greaterthan for a more central location of the fin inductor. In thisembodiment, the concentration of fiux in the vicinity of the couplingloop is effected by relative location of conductive structures of theessential components of the applicator rather than by addition ofauxiliary conductive structure.

As will be seen from the foregoing, by the provision of flux-guides, orflux-gates, or a combination of them, wide latitude in'the determinationor control of the flux density and contour of the fiux path isaiZ-orded. Also such flux-controlling or determining elements may becomprised of portions of the structure of the applicator or by auxiliaryconductive structure.

What is claimed is:

l. A resonant high-frequency heating device comprising conductive wallsdefining a reentrant resonator, metallic fin structure extending rom afirst of said walls and spaced from the other walls to provide theinductance of the resonator, an electrode disposed at and electricallyconnected to the free end of said fin structure in spaced relation tothe resonator Wall opposite said first wall to provide the capacitanceof the resonator, a stationary coupling loop disposed in the resonatorspace which encircles said fin structure for exciting the resonator tocreate an electromagnetic field whose lines of fiux will encircle thefin structure and link with the coupling loop, and means for modifyingthe mutual inductance of said loop and resonator comprising a stationaryconductive plate structure substantially parallel to the fin structureand disposed in said space adjacent said coupling loop to serve as aflux guide to alter the concentration of magnetic flux linked by thecoupling loop.

2. A resonant high-frequency heating device comprising conductive wallsdefining a reentrant resonator, metallic fin structure extending from afirst of said walls and spaced from the other walls to provide theinductance of the resonator, an electrode disposed at and electricallyconnected to the free end of said fin structure in spaced relation tothe tunnel wall opposite said first wall to provide the capacitance ofsaid resonator, a stationary coupling loop disposed in the resonatorspace which encircles said fin structure, and means for modifying themutual inductance of said loop and the resonator, said means comprisingconductive plate structure disposed in said space adjacent said couplingloop and consisting in part of stationary plate structure substantiallyparallel to the fin structure to serve as a flux guide and in partconsisting of structure movable to serve as a flux gate.

3. In a resonant highfrequen-cy heating device, a reent-rant resonatorhaving conductive walls, a metallic fin structure extending from a firstof said walls and spaced from the other walls to provide the inductanceof said resonator, a heating electrode disposed at and electricallyconnected to the free end of said fin structure in spaced relation tothe resonator wall opposite said first Wall to provide the capacitanceof said resonator, a stationary coupling loop disposed in the resonatorspace which oncircles said fin structure for exciting the resonator tocreate an electromagnetic field whose lines of fiux will encircle thefin structure and link with the coupling loop, and conductive platestructure for modifying the mutual inductance of said loop and theresonator comprising at least one movable plate element disposed in saidresonator space adjacent said coupling loop to vary the percentage oftotal magnetic flux linked by the coupling loop.

4. A resonant high-frequency heating device comprising conductive wallsdefining a reentrant resonator, metallic fin structure extending from afirst of said walls and spaced from the other walls to provide theinductance of the resonator, an electrode disposed at and electricallyconnected to the free end of said fin structure in spaced relation tothe resonator wall opposite said first wall to provide the capacitanceof the resonator, a coupling loop disposed in the resonator space whichencircles said fin structure for exciting said resonator to create anelectromagnetic field whose lines of flux will encircle said finstructure and link with said coupling loop, and means for modifying themutual inductance of said loop and said resonator comprising conductiveplate structure disposed in said space adjacent said coupling loopgreases to alter the concentration of magnetic flux linked by-saidcoupling loop.

5. A resonant high-frequency heating device as in, claim 4 in which theconductive plate structure is supported by the resonator wall betweenwhich wall and the fin structure said coupling loop is disposed.

6. A resonant high-frequency heating device as in claim 4 in which saidconductive plate structure is at least in part movable variably tomodify the mutual inductance of the loop and said resonator.

7. A resonant high-frequency heating device as in claim 4 in which saidconductive plate structure extends substantially beyond the edges ofsaid coupling loop and between said loop and an adjacent one of saidWalls to increase the percentage of total flux linked by the couplingloop.

8. A resonant high-frequency heating device comprising conductive wallsdefining a rcentrant resonator, metallic fin structure extending from afirst of said walls and spaced from the other walls to provide theinductance of the resonator, an electrode supported by and connected tothe free end of said fin structure in spaced relation to the resonatorwall opposite said first wall to provide the capacitance of theresonator, a stationary coupling loop disposed in the resonator spacewhich encircles said fin structure for exciting the resonator to createan electromagnetic field whose lines of flux will encircle the finstructure and link with the coupling loop, and means for modifying themutual inductance of said loop and the resonator comprising conductiveplate structure disposed in said space adjacent said coupling loop andbetween said coupling loop and a wall of said resonator to alter theconcentration of magnetic flux linked by said coupling loop.

9. A resonant high-frequency heating device as in claim 8 in which theconductive plate structure is at least in part movable to alter theconcentration of magnetic flux linked by the coupling loop and therebymodify the mutual inductance of the loop and the resonator.

10. In a high-frequency dielectric heating system, the combination of a.power-receiving circuit comprising a resonant applicator havingconductive wall structure forming an electrically conductive housing andhaving therein capacitance and inductance structures respectivelyincluding spaced electrodes of extended area for the heating ofdielectric work disposed in the electric field between the electrodesand an inductance element projecting into the interior of said housingin spaced relation with said wall structure to provide for the magneticfield encircling said inductance element an unobstructed path around thelengthwise thereof, one of said electrodes being formed by orelectrically connected to adjacent wall structure of said housing, asecond of said electrodes being electrically connected to the inwardlyprojecting end of said inductance element in spaced relation to all wallstructure of said housing and being electrically connected with saidwall structure through said inductance element, and means including atleast portions of said wall structure electrically interconnecting saidcapacitance and inductance structures to complete said power-receivingcircuit and affording a high unloaded Q thereof, high-frequencypowersupply means having a power-delivery circuit, a powertransfer loopincluded in said power-delivery circuit and disposed within theunobstructed path of said magnetic field encircling said inductanceelement to provide a loop area of lesser size than the area subtended bythe loop between said inductance element and the opposing wall structureof said housing, and means for modifying the mutual inductance betweensaid power-delivery and power-receiving circuits comprising conductiveplate structure disposed in said space adjacent said coupling loop fordistorting the path of said magnetic field in determination of ratio ofthe magnetic field linking said coupling loop and the magnetic fieldwhich does not link said coupling loop.

11. The system of claim 10 in which said conductive plate structure isat least in part movable with the movable part operable to vary thedegree of localized distortion of said magnetic field to change saidratio.

12. The system of claim in which said conductive plate structureincludes a stationary portion and a mov able portion, and in which saidmovable portion is operable to vary the degree of localized distortionof the magnetic field in the vicinity of said loop and in which saidstationary portion limits said variation to a predetermined range.

13. The system of claim 10 in which said conductive plate structure hasa stationary portion and a movable portion, said stationary portiondistorting said path of said magnetic field to establish a value of saidratio which maintains said mutual inductance coupling substantiallyentirely within the supraoptimum range for all positions of said movableportion and in which said movable portion is adjustable for varying saidratio and said mutual inductance coupling within said supraoptimumrange.

14. The system of claim 10 in which said electrically conductive housinghas therein a conductive walled compartment which itself forms the saidconductive plate structure for distorting the path of the magnetic fieldto predetermine said ratio of the magnetic field linking said couplingloop and the magnetic field which does not link said coupling loop.

15. The system of claim 14 wherein components of said high-frequencypower-supply means are disposed in said compartment and said couplingloop extends from said compartment into the space surrounding saidinductance element and in which said magnetic field is produced.

16. In a high-frequency dielectric heating system, the combination of apowerreceiving circuit comprising a resonant applicator havingconductive wall structure 'forming an electrically conductive housingand having therein capacitance and inductance structures respectivelyincluding spaced electrodes of extended area for the heating ofdielectric work disposed in the electric field between the electrodesand an inductance element projecting into the interior of said housingin spaced relation with said wall structure to provide for the magneticfield encircling said inductance element an unobstructed path around thelengthwise thereof, one of said electrodes being formed by orelectrically connected to adjacent wall structure of said housing, asecond of said electrodes being electrically connected to the inwardlyprojecting end of said inductance element in spaced relation to ail wallstructure of said housing and being electrically connected with saidwall structure through said inductance element, and means including atleast portions of said wall structure electrically interconnecting saidcapacitance and inductance structures to complete said powerreceivingcircuit and aifording a high unloaded Q thereof, high-frequencypower-supply means having a powerdeiivery circuit, a power-transfer loopincluded in said power-delivery circuit and disposed within theunobstructed path of said magnetic field encircling said inductanceelement to provide a loop area of lesser size than the area subtended bythe loop between said inductance element and the opposing wall structureof said housing, said inductance element being asymmetrically disposedwith respect to said housing so as to produce a restriction in the spaceoccupied by said magnetic field in the vicinity of said loop topredetermine the ratio of the magnetic field linking said coupling loopand the magnetic field which does not link said coupling loop.

References Cited in the file of this patent UNITED STATES PATENTS2,504,109 Dakin et al Apr, 18, 1950 2,504,956 Atwood Apr. 25, 19502,504,969 Ellsworth Apr. 25, 1950

